Removing foreign material from coating mix in a circulation system for coating

Abstract

A method and apparatus for separating foreign material from coating mix flow circulating in the machine circulation of a coating station (7) in coaters used for coating a web of paper or board. Coating mix is conveyed from a tank (1) to a coating station, where it is applied to the surface of the web to be coated, and the excess of the coating mix conveyed to the coating station (7) is returned to the tank (1). The coating mix flow incoming from said tank (1) to the coating station is screened by at least one pressure screen (5) and the rejects fraction of the coating mix flow is discharged from the pressure screen (5). A portion of the coating mix flow circulating in the machine circulation is passed through a vibrating screen (13) and the accepts fraction of the vibrating screen is returned back to the machine circulation. The invention offers high efficiency in the removal of elongated foreign matter particles from the machine circulation of coating mix.

What is claimed is: 
     
       1. A method for separating foreign material from coating mix flowing in a circulation system for a coating station used for coating a web of paper or board, the method comprising: conveying coating mix from a storage tank to a coating station where the coating mix is applied to a surface of a web of paper or board to be coated;   returning excess coating mix from the coating station to the storage tank;   screening the coating mix with a pressure screen after the coating mix leaves the storage tank and before the coating mix arrives at the coating station so that an accepts fraction of the coating mix from the pressure screen is conveyed to the coating station and so that a rejects fraction of the coating mix is discharged from the pressure screen; and   screening the rejects fraction of the coating mix discharged from the pressure screen through a vibrating screen so that an accepts fraction of the coating mix from the vibrating screen is conveyed to the storage tank.   
     
     
       2. The method of claim 1, wherein screening of the coating mix in the vibrating screen is performed with a screen having openings which have a diameter of smaller than about 150 μm. 
     
     
       3. The method of claim 2, wherein screening of the coating mix in the vibrating screen is performed with a screen having openings which are not slot-shaped. 
     
     
       4. The method of claim 2, wherein screening of the coating mix in the vibrating screen is performed with a screen having openings which have a diameter of about 100 μm. 
     
     
       5. The method of claim 2, wherein about 5% to about 30% of a volumetric flow of coating mix conveyed to the coating station is screened by the vibrating screen. 
     
     
       6. The method of claim 1, wherein about 5% to about 30% of a volumetric flow of coating mix conveyed to the coating station is screened by the vibrating screen. 
     
     
       7. An apparatus for separating foreign material from coating mix flowing in a circulation system for a coating station used for coating a web of paper or board, the apparatus comprising: a storage tank;   a coating station where coating mix is applied to a surface of a web of paper or board to be coated;   a means for conveying coating mix from the storage tank to the coating station;   a means for returning excess coating mix from the coating station to the storage tank;   a pressure screen positioned to screen the coating mix after the coating mix leaves the storage tank and before the coating mix arrives at the coating station so that an accepts fraction of the coating mix from the pressure screen is conveyed to the coating station and so that a rejects fraction of the coating mix is discharged from the pressure screen; and   a vibrating screen positioned to screen the rejects fraction of the coating mix discharged from the pressure screen so that an accepts fraction of the coating mix from the vibrating screen is conveyed to the storage tank.   
     
     
       8. The apparatus of claim 7, wherein said vibrating screen has openings of a diameter smaller than about 150 μm. 
     
     
       9. The apparatus of claim 8, wherein said vibrating screen has openings which are not slot-shaped. 
     
     
       10. The apparatus of claim 8, wherein said vibrating screen has openings of a diameter of about 100 μm.